The present invention concerns an improvement for supplying an easy and new technique for realizing compact optical attenuators with a reply independent from the wavelength.
The prior art shows the following two documents: U.S. Pat. No. 5,633,974, of Mar. 31, 1995, xe2x80x9cAll fibre attenuatorxe2x80x9d, of Shin-Lo Chia; and WO99/44085 of Sep. 2, 1999, xe2x80x9cOptical fibre attenuators assembled using attenuating fibrexe2x80x9d, of Optotec S.p.A./Egisto Edera; all based on the use of a portion of attenuating fibre, spliced with a standard fibre, and the attenuation independence degree thereof from the wavelength is not very high due to a determined and unavoidable modal interference that arises between the two connections and the blowout joints. Both cited documents describe techniques for limiting the power linked to the upper mode that interferes with the fundamental mode, like the etching of the attenuating fibre portion of the first one or the control of the leak of the blowout joints of the second one, but there always remains a certain interference which may be harmful. Furthermore, the techniques described in said documents allow to realize attenuators with parts of different fibres spliced between each other, which could show problems of reliability, specially the one mentioned in the Chia-document, wherein the diameter of the attenuated fibre is chemically reduced and thus becomes more fragile and subject to the arising of microcracks.
The present invention is based on the use of an optical fibre with a high intrinsic attenuation and with geometric features similar to those of a standard optical fibre and with all consequent advantages; at the same time, it reduces the disadvantages of the two mentioned patents and also allows to obtain a compact attenuator by means of a simple and cheap process.